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Risk Management Activities - Additional Information (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 CAD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Current non-cash assets pledged as collateral for which transferee has right by contract or custom to sell or repledge collateral | $ 49,000,000 | $ 42,000,000 |  |  |
Financial assets pledged as collateral for liabilities or contingent liabilities | 0 | 3,000,000 |  |  |
Derivative, contingent feature, additional collateral requirement | 85,000,000 | $ 51,000,000 |  |  |
Asset-backed debt instruments held | 631,000,000 |  |  |  |
Debt instruments held | $ 1,000,000,000 |  |  |  |
Borrowings subject to floating interest rates | 7.00% | 11.00% |  | 7.00% | 11.00%
Sensitivity analysis used by management for the foreign exchange differences from functional currency | $ 0.03 | $ 0.03 | $ 0.04 |  |
Borrowings | $ 3,361,000,000 | 3,212,000,000 |  |  |
Bond, interest rate | 5.75% |  |  |  |
Target percentage of hedged forecasted foreign operating cash flows over four year period | 60.00% |  |  |  |
Target percentage of hedged forecasted foreign operating cash flows, current period | 90.00% |  |  |  |
Target percentage of hedged forecasted foreign operating cash flows, next year | 70.00% |  |  |  |
Target percentage of hedged forecasted foreign operating cash flows, third year | 50.00% |  |  |  |
Target percentage of hedged forecasted foreign operating cash flows, fourth year | 30.00% |  |  |  |
Change in foreign exchange rates | $ 0 | 0 | 5,000,000 |  |
Maximum exposure to credit risk | 22,000,000 | 5,000,000 |  |  |
Other comprehensive income that will be reclassified to profit or loss, net of tax | (90,000,000) | (19,000,000) | (11,000,000) |  |
Gains (losses) on change in fair value of derivatives | 43,000,000 | 33,000,000 | (29,000,000) |  |
Gains on change in fair value of derivatives | 11,000,000 | 24,000,000 | 3,000,000 |  |
Unrealised loss on change in fair value of derivatives | 2,000,000 |  |  |  |
Unrealised gains on change in fair value of derivatives |  | 6,000,000 | 4,000,000 |  |
Net realized gains on change in fair value of derivatives | 13,000,000 | 18,000,000 |  |  |
Net realized losses on change in fair value of derivatives |  |  | 1,000,000 |  |
Unrealized pre-tax gains (losses) | 1,000,000 | 1,000,000 | 4,000,000 |  |
Within one year |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Other comprehensive income that will be reclassified to profit or loss, net of tax | 72,000,000 |  |  |  |
Long-term debt | Net investment hedges |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Notional amount |  |  |  | $ 370 | $ 370
Forward contract |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Notional amount | $ 150,000,000 |  |  |  |
Bond, forward yield | 1.20% |  |  |  |
Bond Lock Agreement [Member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Notional amount | $ 75,000,000 |  |  |  |
Commodity Price Risk - Proprietary Trading [Member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Value at risk | 1,000,000 | 1,000,000 | 2,000,000 |  |
Commodity Price Risk - Generation [Member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Value at risk | 12,000,000 | 25,000,000 | 18,000,000 |  |
Commodity Price Risk - Generation - Mark To Market Value [Member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Value at risk | 15,000,000 | 8,000,000 | $ 13,000,000 |  |
Interest rate risk | Interest Rate Exposure [Member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Borrowings | $ 150,000,000 |  |  |  |
Interest rate risk | Interest rate swap |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Notional amount |  |  |  | 150 |
Borrowings |  |  |  | $ 400 |
Interest rate risk | Interest rate swap | Fixed interest rate [member] |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Derivate, fixed interest rate | 0.94% |  |  |  |
Letter of credit |  |  |  |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |  |  |  |
Collateral posted on derivative liabilities | $ 163,000,000 | $ 112,000,000 |  |  |

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